Load tap changing apparatus

ABSTRACT

The rotatable contact carrier of a tap changer dial switch carries both tap changer movable contacts and an odd-even switch conductive plate having a plurality of arcuately spaced radial projections which engage first and second finger contacts to form first and second normally closed pairs of contacts connected in series with the tap changer first and second movable contacts respectively. Each time the contact carrier is rotated to accomplish a tap change, one contact finger falls between adjacent radial projections on the conductive switchplate to open the pair of contacts in series with the corresponding movable tap changer contact before the movable tap changer contact disengages a stationary tap changer contact and engages a succeeding radial projection to reclose the pair of contacts after the movable tap changer contact engages the succeeding stationary tap changer contact.

United States Patent [72] Inventor Carl C. Whitman Mukwonago, Wis.

[21] Appl. No. 87,752

[22] Filed Nov. 9, 1970 [45] Patented Oct. 12, 1971 [73] Assignee Allis-Chalmers Manufacturing Company Milwaukee, Wis.

[54] LOAD TAP CHANGING APPARATUS 11 TC; 323/435 R [56] References Cited UNITED STATES PATENTS 2,947,838 8/1960 Ureda 200/11 TC 3,066,208 1 H1962 Fannon, Jr. et al. 200/1 1 TC Primary Examiner-J. D. Miller Assistant ExaminerA. D. Pellinen Attorneys-Lee l-l. Kaiser, Thomas F. Kirby and Robert B.

Benson ABSTRACT: The rotatable contact carrier of a tap changer dial switch carries both tap changer movable contacts and an odd-even switch conductive plate having a plurality of arcuately spaced radial projections which engage first and second finger contacts to form first and second normally closed pairs of contacts connected in series with the tap changer first and second movable contacts respectively. Each time the contact carrier is rotated to accomplish a tap change, one contact finger falls between adjacent radial projections on the conductive switchplate to open the pair of contacts in series with the corresponding movable tap changer contact before the movabie tap changer contact disengages a stationary tap changer contact and engages a succeeding radial projection to reclose the pair of contacts after the movable tap changer contact engages the succeeding stationary tap changer contact.

PATENIEDUCT 12 I971 3. 6 l 2 7 86 v saw a or 5 -QW MM Pmm 12 1911 SHEET 4 OF 5 PATENTEDUCT 12 Ian 3,612.7 86

SHEET .SUF 5 OPEN I ODD N CLOSED SWITCH CONTACTS L N 1:, 6 7

' CURRENT THROUGH I scRs 8OAND8| i I 5498/2 t, 3 7

V OPEN TAP CHANGED CONTACTS t gin/(3.8%

GATE CURRENT To 2 (I): scRs AND I I 80x8 4 I Q N BA TIME t6 LOAD TAP CHANGING APPARATUS This invention relates to load tap changing apparatus such as tap changing voltage regulators and in particular to such apparatus which change taps without arcing at the tap changer contacts.

Arcing between movable and stationary contacts during each switching cycle of a load tap changing apparatus such as a voltage regulator generates combustible gases within the casing, contaminates the oil and causes pitting and erosion of the contacts. Such arcing necessitates the use of a breather on the regulator casing which permits moisture, dust and other foreign material to enter the casing through the breather and further contaminate the oil. Further, the breather prevents installation of a voltage regulator in an underground location. The pitting and erosion and wear of the tap changer contacts necessitates taking the regulator out of service periodically for maintenance and, eventually, replacement of the contacts. Inspection and maintenance of the contacts may require untanking of the oil filled regulator, and this is time consuming and costly since it requires removal of cover bolts and lifting the heavy unit out of the tank with a crane or portable inspection jack- It is an object of the invention to provide improved load tap changing apparatus including an odd-even switch having contacts in series with the tap changer contacts which eliminates arcing between the tap changer contacts, substantially extends their useful operating life, and minimizes maintenance required of the tap changer and the time that it is out of service.

It is a further object of the invention to provide improved load tap changing apparatus for a voltage regulator including an odd-even switch having contacts in series with the tap changer contacts which permits elimination of the conventional breather and installation of the regulator in an underground location, thereby avoiding the problem of moisture, dust and foreign particles which entered into casings of prior art regulators through such a breather.

Another object of this invention is to provide a dial switch tap changer embodying an odd-even switch which opens a pair of contacts in series with the movable tap changer contact before the movable tap changer contact disengages from a stationary contact and recloses the odd-even switch contacts after the tap change has been completed, thereby preventing interruption of load current by the tap changer contacts. An object of one embodiment of the invention is to provide such a dial switch tap changer embodying an odd-even switch wherein the contacts of the odd-even switch are moved in a series of steps and are latched after each step to assure that they open before and close after the tap changer switch contacts. Still another object of the invention is to provide such a dial switch tap changer embodying anodd-even switch wherein the odd-even switch is actuated by same mechanism that rotates the tap changer movable contacts.

These and other objects and advantages of the invention will be more readily apparent from the following detailed description when considered in conjunction with the accompanying drawing wherein:

FIG. 1 is a schematic circuit diagram of a load tap changing voltage regulator embodying the invention;

FIG. 2 is a view of the tap changer of the embodiment of FIG. 1 taken along line 11-" of FIG. 3;

FIG. 3 is a side view of the tap changer shown in FIG. 2;

FIG. 4 is a view of the odd-even switch;

FIG. 5 is a partial side view similar to FIG. 3 illustrating a single stationary and a single movable contact;

FIG. 6 is a perspective view illustrating the mechanism for actuating the rotatable contacts;

FIGS. 7a through 7f schematically illustrate the sequence of operations in changing taps in the apparatus of FIGS. 1-7;

FIGS 8a through 8d schematically illustrate the relative timing of the currents and the switch contacts during a single tap change; and

FIG. 9 illustrates an embodiment of the invention wherein the contact carrier is moved in a series of steps during each tap change and is latched after each step.

The combined dial switch and odd-even switch of the invention will be described as embodied in the arcless load tap changing voltage regulator using thyristors to interrupt current in the load circuits during tap changes disclosed in the copending U.S. application of Frederick A. Stich Ser. No. 889,369 filed Dec. 3, 1969, entitled Arcless Load Tap Changing Apparatus and having the same assignee as this invention. The mechanism which drives the movable contacts between tap positions and also operates the odd-even switch is similar to the type disclosed in U.S. Pits. Nos. 2,8l 1,595 to A. H. Baguhn and 2,841,662 to William Sealey, to which reference is made for details of construction.

Referring to FIG. 1, a load tap changing voltage regulator of the autotransformer type has a source terminal S adapted to be connected to an unregulated alternating current power supply, an exciting, or shunt winding 10 connected between the source terminal S and ground, and a series winding 11 having a plurality of taps connected to stationary contacts 1-8 and the neutral stationary contact N of a dial-type load tap changing switch. The regulator provides a regulated voltage to a load connected to bushing L, and the ends of series winding 11 are connected to the stationary contacts 12 and 14 of a reversing switch 15 having a movable contact 16 connected to tlie neutral stationary contact N and to exciting winding 10 and adapted to alternatively engage stationary contact 12 or 14 to connect series winding 11 in bucking or boosting rela tion to exciting winding 10.

The tap changer movable, or rotatable contacts 20 and 21 are adapted to sequentially engage the stationary contacts 1-8 and N and are connected through the windings 23 and 24 of a preventive autotransformer, or reactor 26 to the load bushing L over conductors 27 and 28 respectively. The load tap changer is preferably a dial switch having the stationary contacts 1-8 and N arranged in a circle (see FIGS. 2, 3 and 7) and having the movable contacts 20 and 21 rotatable together about a common axis into sequential engagement with the sta tionary contacts and being adapted to assume both nonbridging tap positions, wherein both rotatable contacts 20 and 21 engage the same stationary contact, and bridging tap positions wherein they engage adjacent stationary contacts. Stationary contacts 1-8 and N are arcuately spaced apart and each comprises a generally U-shaped metallic contact 30 (see FIGS. 3 and 5) having one leg brazed to studs 31 extending through an insulating panel 32 and electrically connected to the corresponding winding tap. Each stationary contact l-8 and N may extend arcuately over approximately 30. Each movable contact 20 and 21 has a pair of opposed resilient contact fingers 34 (see FIGS. 2, 3 and 5) at its radially outer end adapted to engage opposite sides of each stationary contact 1-8 and N. The movable contacts 20 and 21 are mounted in arcuately spaced relation on an insulating contact carrier 37 which is rotatable about a metallic collector hub 38 affixed to insulating panel 32 by metallic studs 39. The movable contacts 20 and 21 are preferably spaced by an angle such as 20 which is equal to the angle through which they move during each tap change and is greater than the angle between adjacent stationary contacts 30 to permit them to assume bridging tap positions.

Rotatable contact 20 has a pair of opposed resilient fingers 41 at its radially inner end which maintain continuous electrical engagement with collector hub 38 as contact carrier 37 rotates. Rotatable contact 21 has a pair of opposed resilient fingers 43 (see FIG. 2) at its radially inner end which maintain continuous electrical engagement with metallic collector ring 44 affixed to insulating panel 32.

The ends of the legs of a generally U-shaped mounting frame 51 are affixed to insulating panel 32. A stationary circular index plate 53 having circumferentially spaced notches 54 about its periphery is affixed by bolts to mounting frame 51 in spaced parallel relation to insulating panel 32. A main shaft 55 extends axially through index plate 53 and is afiixed to mounting frame 51. A sprocket wheel 57 having external teeth is rotatable about main shaft 55 and is driven by a chain drive 58 actuated by a motor sprocket 59 mounted on the shaft of reversible motor M (see FIG. 3) which is energized to rotate sprocket wheel 57, and thus rotate movable contacts and 21, in a direction to lower or raise the voltage supplied over conductors 27 and 28 to load bushing L. The motor M may be controlled in known manner to rotate sprocket wheel 57 at uniform speed to change taps in a direction to maintain the voltage supplied over conductors 27 and 28 to load bushing L within a desired voltage bandwidth.

An actuating, or interlock disk 60 rotatable about main shaft 55 is coaxial with and disposed between sprocket wheel 57 and contact carrier 37 and carries a pair of driving pins 61 extending parallel to the axis thereof which fit within complementary grooves 63 in contact carrier 37 so that interlock disk 60 rotates with contact carrier 37, and thus actuates movable contacts 20 and 21. Sprocket wheel 57 is connected to interlock disk 60 by two spring assemblies 65 (see FIG. 3) positioned symmetrically about main shaft 55. One end of each spring assembly 65 is attached to a pin 67 affixed to sprocket wheel 57 and its other end is attached to a pin 68 affixed to interlock disk 60 so that spring assemblies 65 are biased in both directions of rotation of sprocket wheel 57.

Actuating disk 60 is normally held stationary by a latch 70 which is pivotally attached to actuating disk 60 and extends through an arcuately elongated aperture 71 (see FIG. 6) in sprocket wheel 57 and engages one of the notches 54 in index plate 53. At the beginning of a tap change, sprocket wheel 57 is in a balanced position relative to actuating disk 60 so that neither spring assembly 65 is loaded and latch 70 is held in a notch 54 in index plate 53 by spring means (not shown). As sprocket wheel 57 is rotated at slow uniform speed by the motor M and chain drive 58, the spring assemblies 65 are biased since like ends of the spring assemblies 65 are attached to the rotating sprocket wheel 57 while the other ends are attached to actuating disk 60 which is prevented from turning by the engagement of latch 70 in a notch 54. After a predetermined rotation of sprocket wheel 57, a cam surface 73 partially defining aperture 71 lifts latch 70 from the notch 54 in index plate 53, thereby permitting the biased spring assemblies 65 to rapidly rotate actuating disk 60, and thus snap contact carrier 37 and movable contacts 20 and 21 with a rapid motion to the succeeding tap position.

ODD-EVEN SWITCH The voltage regulator described hereinbefore is known and depends upon high speed of movement of the rotatable contacts 20 and 21 to effect rapid extinction of the are drawn between the stationary and movable contacts. The arcless load tap changing apparatus described herein having thyristors to interrupt current in the load circuits permits movable contacts 20 and 21 to engage or disengage from the stationary contacts 1-8 and N without arcing regardless of the speed at which such movable contacts 20 and 21 are actuated and while supplying current continuously to the load.

An odd-even switch 75 incorporated in the voltage regulator and operated by the mechanism described above has a pair of normally closed contacts AB (see FIG. 1) in series with conductor 27 and a pair of normally closed contacts AD in series with conductor 28. Contacts AB and AD are between load bushing L and the movable tap changer contacts 21 and 20 respectively and open before a movable contact 20 or 21 disengages from a stationary contact 1-8 or N and close after the movable contact 20 or 21 engages the adjacent stationary contact in a manner analogous to a load transfer switch so that the tap changer contacts do not interrupt load current and also so that, even when one rotatable contact 20 or 21 is being moved between adjacent tap changer stationary contacts, the current is being supplied continuously to the load through the other rotatable tap changer contact 20 or 21 and the other pair of odd-even switch contacts. Thus a first load circuit is formed by the series arrangement of movable contact 20, reactor winding 23, conductor 28 and odd-even switch contacts AD, and a second load circuit is formed by the series arrangement of movable contact 21, reactor winding 24, conductor 27, and odd-even switch contacts AB. The term odd-even" connotes that one pair of contacts AB or AD open when the movable contacts 20 and 21 are actuated to an odd-numbered tap position and the other pair of contacts AB or AD open when the movable contacts 20 and 21 are actuated to an even-numbered tap position.

Controllable semiconductor means which preferably comprise a pair of oppositely poled thyristors, or silicon controlled rectifiers, termed SCRs and 81 are connected across the first and second load circuits and in shunt to the series arrangement of the odd-even switch contacts AB and Ad, and gating signal control means (see FIG. 1) are provided to gate SCRs 80 and 81 to the conductive state before the odd-even switch contacts AB or AD open and to remove gating power from SCRs 80 and 82 subsequent to opening of odd-even contacts AB or AD so that the current carried by the tap changer rotatable contact 20 or 21 being switched is interrupted by SCR 80 or 81 at the next current zero and the odd-even switch contacts AB and AD never interrupt current.

The electrically commoned contacts of the pairs of contacts AB and AD of odd-even switch 75, shown schematically in FIG. 1 preferably include an annular switch plate A affixed to and rotatable with contact carrier 37 and two pairs of electrically commoned contact fingers A and A" one of which is always in engagement with switch plate A as it rotates. Switch plate A has the same number of arcuately spaced radial projections 85 at its outer periphery as the tap changer has stationary contacts 1-8 and N, i.e., nine projections 85. The radial projections 85 may be in the same radial planes as the stationary contacts 1-8 and N but subtend a somewhat smaller arc, preferably approximately 26.

The pair of normally closed contacts AB of odd-even switch 75 shown schematically in FIG. 1 preferably also includes a pair of resilient fingers B attached to a metallic contact support 88 which is affixed by metallic studs 89 that extend through panel 32 and are electrically connected to conductor 27. Contact fingers B normally engage opposite sides of a radial projection 85 adjacent the trailing edge thereof in a clockwise direction of rotation, and they fall between adjacent projections 85 so that they are disengaged from odd-even switch plate A when plate A is rotated (e.g., clockwise from the position shown in FIG. 2) to first open contacts AB of the odd-even switch 75 before rotatable tap changer contact 21 is disengaged from a stationary contact when it is being actuated. The pair of contacts AD of odd-even switch 75 shown schematically in FIG. 1 preferably also includes a pair of resilient contact fingers D arcuately spaced from contact fingers B and attached to a similar metallic contact support 88 which is affixed by metallic studs (not shown) that extend through panel 32 and are electrically connected to conductor 28. Contact fingers D normally engage opposite sides of a radial projection 85 adjacent the leading edge thereof in a clockwise direction of rotation and they fall between adjacent projections 85 so that they are disengaged from plate A when plate A is rotated (e.g., counterclockwise from the position shown in FIG. 2) to first open odd-even switch contacts AD before tap changer rotatable contact 20 is disengaged from a stationary contact when it is being actuated to a succeeding tap position.

The pairs of resilient contact fingers A and A" (which comprise the electrically commoned contacts A of both pairs of contacts AB and AD of odd-even switch 75) are affixed to the same metallic contact support 96. Contact support 96 is attached by metallic studs (not shown) which extend through insulating panel 32 and are electrically connected to load bushing L. Opposed contact fingers A normally engage opposite sides of a projection 85 of switch plate A and opposed contact fingers A" engage opposite sides of the same projection 85 when rotatable contacts 20 and 21 are on the same tap changer stationary contact. Contact fingers A and A" are arcuately spaced through an angle greater than that between adjacent projections 85 so that one pair of fingers A or the other A" is always in engagement with switch plate A even when it is being rotated. Further, contact fingers B and D are arcuately spaced so that contact fingers B engage a radial projection 85 adjacent its trailing edge (assuming the subsequent rotation to be clockwise as seen in FIGS. 2 and 4) while the contact fingers D engage a projection 85 adjacent its leading edge. The angle subtended by each projection 85 is preferably approximately 26 which is greater than the angle through which movable contacts 20 and 21 are rotated during each tap change, i.e., approximately 20, thereby assuring that either contact fingers B or contact fingers D are in engagement with a projection 85 on switch plate A at all times. The stationary contact fingers A, A", B and D cooperates with rotatable switch plate A to form the sets of odd-even contacts shown schematically at AB and AD in FIG. 1, and this structure assures that one set of odd-even switch contacts AB or AD is always closed so that one load circuit is always completed and the current to load bushing L is never interrupted.

OPERATION OF ODD EVEN SWITCH FIGS. 7a7f schematically illustrate operation of odd-even switch 75. The schematic circuit diagram with the odd-even contacts represented #is shown at the left of each FIG. 7a-7f and the physical embodiment is shown to the right thereof. FIG. 7a represents that tap changer rotatable contacts 20 and 21 both engage stationary contact 7 and that odd-even switch contact fingers B engage the trailing edge of one projection 85 on switch plate A (assuming the succeeding tap change to be clockwise) and contact fingers D engage the leading edge of another projection 85 so that both contact pairs AB and AD are closed. FIG. 75 represents that contact carrier 37 has been rotated sufficiently in a clockwise direction (approximately 5) to disengage contact fingers B from switch plate A but not far enough to disengage tap changer rotatable contact 21 from stationary contact 7, thereby opening odd-even switch contact pair AB as schematically illustrated in the left portion of FIG. 7b. FIG. 70 illustrates that the gating signals to thyristors 80 and 81 are removed after contacts AB open as described hereinafter so that the thyristors 80 or 81 interrupt the current in the second load circuit at current zero and the contacts AB do not interrupt current. Since odd-even switch contacts AB are open, movable contact 21 may disengage stationary contact 7 without interrupting current, and rotatable contact 21 disengages stationary contact 7 after approximately 10 of rotation of contact carrier 37. FIG. 7d illustrates that contact carrier 37 has been rotated sufficiently further in the clockwise direction (approximately 16) to engage rotatable tap changer contact 21 with the succeeding tap changer stationary contact 8 but not sufficiently far to engage odd-even switch finger contacts B with the succeeding radial projection on switch plate A, thereby assuring that thetap change takes place while odd-even switch contacts AB are open and load current is not interrupted by tap changer movable contact 21. It will be appreciated that during such sequence of operations, odd-even switch contact fingers D has moved from the leading to the trailing edge of a radial projection 85 but has remained in continuous engagement therewith so that load current flows to load bushing L through the first load circuit including tap changer rotatable contact 20 and odd-even switch contacts AD while movable contact 21 was being rotated to the succeeding tap changer stationary contact 8.

Contact fingers B engage the succeeding radial projection 85 on switch plate A after approximately 18 of rotation of contact carrier 37 as represented in FIG. 72 to reclose contacts AB. During the tap change finger contacts A" have moved from the leading to the trailing edge of a radial projection 85 but have maintained continuous engagement with this projection 85.

It will be appreciated that in alternative embodiments of the invention, the actuating member 60 can be omitted and the latch means 70 mounted on, and the spring assemblies 65 connected to, an operating member which carries the movable contacts 20 and 21 and switch plate A.

SCR GATING CONTROL Means responsive to the current flow in both of the load circuits supply triggering signals to SCRs and 81 and are responsive to the opening of either load circuit to remove the triggering signals after a predetermined time delay. The primary winding of a current transformer (see FIG. I) of gating signal control means 82 is connected in series with conductor 27 and odd-even switch contacts AB, and the primary winding of a current transformer 200 is connected in series with conductor 28 and odd-even switch contacts AD. Current transformer 100 has a pair of secondary windings 101 and 102 which are connected to full wave bridge rectifiers 203 and 104 respectively. Similarly current transformer 200 has a pair of secondary windings 201 and 202 which are connected to bridge rectifiers 103 and 204 respectively. Current transformers 100 and 200 provide gating power for SCRs 80 and 81 and also indicate whether the contacts AB and AD of the oddeven switch 75 are open or closed.

When odd-even switch contacts AB and AD are closed, bridge rectifier 104 provides gating power over conductors 109 and 110 to SCR 80 and bridge rectifier 204 provides gating power over conductors 209 and 210 to SCR 81. When either pair of odd-even switch contacts AB or AD is opened, gating power is maintained momentarily to both SCRs 80 and 81 for at least one full cycle after the odd-even switch contacts open so that the current carried by the rotatable tap changer contact 20 or 21 being actuated is interrupted by SCR 80 or 81 at the current zero following removal of gating power from the SCRs. Current then flows to load bushing L through the other load circuit, i.e., through the other rotatable tap changer contact 20 or 21 and the other pair of odd-even switch contacts AB or AD.

A gating current supply capacitor connected in series with a resistance 116 across conductors 109 and 110 is charged by bridge rectifier 104 when odd-even switch contacts AB and AD are closed and supplies gating power to SCR 80 after odd-even switch contacts AB open and bridge rectifier 104 is deenergized. Similarly, a gating current supply capacitor 215 connected in series with a resistance 216 across conductors 209 and 210 is charged by bridge rectifier 204 when odd-even switch contacts AB and AD are closed and supplies gating power to SCR 81 after oddeven switch contacts AD opens and bridge rectifier 204 is deenergized.

Industry standards require that a distribution voltage regulator be capable of carrying 64 times rated current under short circuit conditions without failure, for example, 64 times 200 amperes rated current. It a short circuit occurs on the voltage regulator in the condition shown in FIGS. 1 and 7a when both odd-even switch contact pairs AB and AD are closed, the short circuit current flows through the contacts AB and AD of the odd-even switch 75 so that SCRs 80 and 81 are not subjected to short circuit conditions.

The load tap changing apparatus precisely controls the time intervals after opening of odd-even switch contacts ABor AD that gating power is removed from SCRs 80 and 81, thereby minimizing the possibility of subjecting the SCRs to short circuit currents when the odd-even switch contacts AB or AD are open and permitting use of SCRs of lower rating. Timing capacitors I20, 121, 220 and 221 associated with bridge rectifiers 103, 104, 203 and 204 respectively regulate the time interval after opening of an odd-even switch contact pair A8 or AD before gating power is removed from SCRs 80 and 81. Timing capacitors 120, I21, 220 and 221 are charged through resistances 125, 126, 225 and 128 respectively from bridge rectifiers 103, 104, 203 and 204 when odd-even switch contact pairs AB and AD are closed.

SHUNT REGULATOR FOR SCR 80 A shunt regulator provided for SCR 80 includes a control element which preferably is a shunting transistor 130 having its emitter-collector junction connected across conductors 109 and 110, a reference voltage zener diode 131 connected across the base-collector junction of shunting transistor 130 and also connected in series with a base resistance 134 across conductors 109 and 110, and a voltage dropping resistance 132 in series with conductor 110 between bridge rectifier 104 and the shunt regulator. The output voltage from the shunt regulator remains constant because the current through shunting transistor 130 changes as the input voltage to the shunt regulator or the gating current changes. The base-collector voltage of shunting transistor 130 is held constant by reference voltage zener diode 131, and thus any change in input voltage from bridge rectifier 104 appears across the base resistance 134 and changes the potential applied to the baseemitter junction of shunting transistor 130 so that its collector current which flows through voltage dropping resistance 132 varies in a direction to maintain the shunt regulator output voltage constant. Changes in gating current to SCR 80 vary the voltage drops across a dropping resistor 135 connected between the shunt regulator and the gate of SCR 80, and the shunt regulator in combination with voltage dropping resistor 135 assures that the gate of thyristor 80 never dissipates more than a predetermined amount of power, preferably 2 watts.

Gating current is maintained to SCR 80 for at least one full cycle after odd-even switch contacts AB or AD open to assure that the current through the tap changer movable contact 20 or 21 being switched is interrupted by SCR 80 (or SCR 81) at current zero and not by the odd-even switch contacts. Timing capacitor 121 assures that gating current flows to SCR 80 for a predetermined interval after contacts AB open, and timing capacitor 120 assures that such gating current to SCR 80 flows for a predetermined interval after contacts AD open.

Shunting transistor 130 is biased in the active region when zener diode 31 breaks down and either a first holding transistor 140 or a second holding transistor 141 is biased ON, thereby permitting gating current to flow over conductors 109 and 110 to SCR 80. When either first or second holding transistor 140 or 141 is biased OFF, shunting transistor 130 is biased into saturation and shunts gating current from SCR 80, thereby permitting SCR 80 to interrupt current carried by the tap changer movable contact 20 or 21 at current zero.

When odd-even switch contacts AB and AD are closed, first holding transistor 140 is biased ON by the charge on timing capacitor 120 which is coupled through a diode 142 and a resistance 143 to the base of first holding transistor 140, and second holding transistor 141 is biased ON by the charge on timing capacitor 121 which is coupled through a diode 144 and a resistance 145 to the base of second holding transistor The collector of first holding transistor 140 is coupled through a resistance 147 to bridge rectifier 104 and timing capacitor 121. When first holding transistor 140 is biased ON, the voltage of bridge rectifier 104 causes current to flow through resistance 147 and the collector-emitter junction of first holding transistor 140 to conductor 109, and the potential impressed across the base-emitter junction of shunting transistor 130 is determined by the input voltage to the shunt regulator which controls the voltage drop across base resistance 134. Shunting transistor 130 is biased OFF until the input voltage to the shunt regulator is sufficiently high to break down zener diode 131. When zener diode 131 conducts, the shunting transistor 130 is biased in the active region so that gating current can flow to SCR 80. When first holding transistor 140 is biased OFF, the voltage of rectifier bridge 104 (or of timing capacitor 121) causes current to flow through resistance 147, a coupling diode 148, and base resistance 134 to conductor 109, and the increased voltage drop across base resistance 134 biases shunting transistor 130 into saturation which shunts, or cuts off gating current from SCR 80 and rapidly discharges gating current supply capacitor 115.

The collector of second holding transistor 141 is coupled through a resistance 149 to timing capacitor 120. When second holding transistor 141 is biased ON, the charge on timing capacitor causes current to flow through resistance 149 and the collector-emitter junction of second holding transistor 141 to conductor 109. When second holding transistor 141 is biased OFF, the voltage on timing capacitor 120 causes current to flow through resistance 149, a coupling diode 150, and base resistance 134 to conductor 109, and the increased voltage drop across base resistance 134 biases shunting transistor into saturation to shunt gating current from SCR 80. First and second holding transistors and 141 together with coupling diodes 148 and 150 may be considered an OR circuit which biases shunting transistor 130 in the active region when either first or second holding transistors 140 or 141 is ON, thereby permitting gating current to flow to SCR 80.

When odd-even switch contacts AB open, current transformer 100 and bridge rectifier 104 are deenergized, but gating current supply capacitor 115 discharges through the series arrangement of a diode 152, dropping resistance 132, three series diodes 153, 154 and 155, resistance 135, and a diode 157 to supply gating current to SCR 80 after contacts AB open. Further, timing capacitor 121 discharges through diode 144, resistance 145 and a resistance 160 coupled between the base of second holding transistor 141 and conductor 109, and the voltage drop across resistance 160 temporarily keeps second holding transistor 141 biased ON. The charge on timing capacitor 121 decays, and a predetermined interval after contacts AB open, second holding transistor 141 is biased OFF. The decrease in voltage across timing capacitor 121 as it discharges is reflected through resistance 147 to the collector of first holding transistor 140, thereby biasing it OFF after a predetermined time delay. The biasing of either first or second holding transistors 140 and 141 OFF increases the current through base resistor 134 and thus increases the base-emitter potential applied to shunting transistor 130, thereby biasing it into saturation and shunting gating current from SCR 80. In this manner, the gating current to SCR 80 is clipped, or truncated a predetermined interval, preferably 25 milliseconds, after odd-even switch contacts AB open and also a predetermined interval before the movable tap changer contact disengages the stationary contact. This assures that SCR 80 remains ON after contacts AB open so that contacts AB never interrupt current, and it also assures that the probability of SCR 80 being subjected to fault current when it is gated ON is minimal.

When odd-even switch contacts AD open, current transformer 200 and bridge rectifier 103 are deenergized, but timing capacitor 120 discharges through a diode 142, resistance 143, and a resistance 162 coupled to the base of first holding transistor 140, thereby temporarily biasing first holding transistor 140 ON. The voltage on timing capacitor 120 decays, and first holding transistor 140 is biased OFF a predetermined interval after odd-even switch contacts AD open. The decrease of voltage on timing capacitor 120 is reflected through resistance 149 to the collector of second holding transistor 141 and biases it OFF after a desired time delay. When either first or second holding transistors 140 and 141 is biased OFF, the current through base resistance 134 and thus the potential applied to the base-emitter junction of shunting transistor 130 increases and biases it into saturation. When shunting transistor 130 saturates, gating current is shunted from SCR 80 so it interrupts current flowing through tap changer movable contact 20 or 21 at the first current zero. It will be noted that the truncating, or clipping of gating cur rent to SCR 80 occurs a predetermined interval after contacts AD open and also a predetermined interval before the tap changer contacts disengage to assure that the probability is minimal of SCR 80 being subjected to fault current while gated ON.

SHUNT REGULATOR FOR SCR 81 The shunt regulator for SCR 81 is similar to that for SCR 80 and includes a control element which preferably is a shunting transistor 230 having its emitter-collector junction connected across conductors 209 and 210, a reference voltage zener diode 231 connected across the base-collector junction of shunting transistor 230 and also connected in series with a base resistor 234 across conductors 209 and 210, and a voltage dropping resistance 232 in series with conductor 210 between bridge rectifier 204 and the shunt regulator. The voltage from such shunt regulator remains constant because the current through shunting transistor 230 changes as the input voltage to the shunt regulator or the gating current to SCR 81 changes.

The base-collector voltage of shunting transistor 230 is held constant by reference voltage zener diode 231, and thus any change in input voltage from bridge rectifier 204 appears across the base resistance 234 and changes the potential apthat its collector current, which flows through dropping re sistance 232, varies in a direction to maintain the shunt regulator output voltage constant. A change in gating current to SCR 81 varies the voltage drop across a dropping resistance 235 connected between the shunt regulator and the gate of SCR 81, and the shunt regulator in combination with dropping resistance 235 assures that the gate of SCR 81 never dissipates more than a predetermined amount of power, preferably 2 watts.

Gating current is maintained to SCR 81 for at least one full cycle after odd-even switch contacts AB or AD open to assure that the current is interrupted by SCR 81 (or by SCR 80) and not by the odd-even switch contacts. Timing capacitors 220 and 221 assure that gating current flows to SCR 81 for a predetermined interval after contacts AB and AD respectively open and is cut ofi a predetermined interval before the tap changer contacts disengage.

Shunting transistor 230 is biased into the active region when zener diode 231 breaks down and either a first holding transistor 240 or a second holding transistor 241 is biased ON, thereby permitting gating current to flow over conductors 209 and 210 to SCR 81. When either a first or a second holding transistor 240 and 241 is turned OFF, shunting transistor 230 is biased into saturation and shunts gating current from SCR 81, thereby permitting SCR 81 to interrupt the current carried by the tap changer movable contact 20 or 21 being rotated at current zero.

When odd-even switch contacts AB and AD are closed, first and second holding transistors 240 and 241 are biased ON by the charge on timing capacitors 220 and 221 respectively.

When odd-even switch contacts AD open, current transformer 200 and rectifier 204 are deenergized, but gating current supply capacitor 215 discharges through the series arrangement of a diode 252, dropping resistance 232, three series diodes 253, 254 and 255, dropping resistance 235 and a diode 257 to supply gating current to SCR 81 after contacts AD open. Further, timing capacitor 221 discharges through a diode 244, a resistance 245, and a resistance 260 connected to the base of second holding transistor 241 and also through resistance 247 in series with saturated first holding transistor 240, and the voltage drop across resistance 260 temporarily maintains second holding transistor 241 biased ON. The charge on timing capacitor 221 decays, and second holding transistor 241 is biased OFF a predetermined interval after contacts AD open. The decrease in voltage across timing capacitor 221 is also reflected through resistance 247 to the collector of first holding transistor 240, thereby biasing it OFF after a time delay. The turning of either first or second holding transistors 240 or 241 OFF increases the flow of current through base resistance 234 and thus increases the baseemitter potential applied to shunting transistor 230, thereby biasing it into saturation and shunting gating current from SCR 81. The gating current to SCR 81 is thus truncated, or clipped a predetermined interval, preferably 25 milliseconds, after odd-even switch contacts AD open and also a predetermined interval before the tap changer contacts disengage.

When odd-even switch contacts AB open, current transformer 100 and bridge rectifier 203 are deenergized, but timing capacitor 220 discharges through a diode 242, resistance 243 and a resistance 262 connected to the base of first holding transistor 240, thereby temporarily biasing first holding transistor 240 ON. The voltage on timing capacitor 220 decays, and first holding transistor 240 is biased OFF a predetermined interval after odd-even switch contacts AB open. The decrease of voltage on timing capacitor 220 is reflected through a resistance 249 to the collector of second holding transistor 241 and biases it OFF after a time delay. When either first or second holding transistors 240 and 241 is biased OFF, the current flow through base resistor 234 increases and thus the potential applied to the base-emitter y junction of shunting transistor 230 increases and biases it into plied to the base-emitter 11111611011 of shunting transistor 230 so saturation. When shunting transistor 230 saturates, gating current is shunted from SCR 81 so it (or SCR interrupts current through the tap changer movable contact 20 or 21 being switched at the next current zero. The truncating, or clipping of gating current to SCR 81 occurs a predetermined interval after contacts AB open and also a predetermined interval prior to disengagement of the tap changer contacts to assure that the probability is minimal that SCR 81 will be subjected to fault current while gated ON.

OPERATION Assume the condition shown in FIG. 7a with both tap changer rotatable contacts 20 and 21 engaging tap changer stationary contact 7 and both odd-even switch contacts AB and AD closed. In this condition, the load current flows through both rotatable tap changer contacts 20 and 21, through both preventive autotransformer windings 23 and 24, and both conductors 27 and 28 to load bushing L so that both current transformers 100 and 200 are energized and both SCRs 80 and 81 are gated ON. If a short circuit occurs on the voltage regulator in the condition shown in FIG. 7a, the short circuit current flows through the odd-even switch contacts AB and AD so that SCRs 80 and 81 are not subjected to short circuit current.

Assume further that the voltage impressed upon load bushing L is out of the voltage bandwidth and that the regulator control circuit (not shown) energizes motor M to actuate tap changer retractable contact 21 to the succeeding stationary contact 8. Chain drive 58 rotates sprocket wheel 57 while latch 70 is retained within a notch 54 on index plate 53 to prevent actuating disk 60 and contact carrier 37 from moving, thereby loading one spring assembly 65. After a predetermined rotation of sprocket wheel 57, cam surface 73 on sprocket wheel 57 lifts can 70 out of its notch 54 in index plate 53 and thus frees actuating disk 60 for rotation. Afier rotation of contact carrier 37 and switch plate A through a first predetermined angle (approximately 5 and assumed to be clockwise), contact fingers B are disengaged from the corresponding projection 85 on switch plate A, as illustrated in FIG. 7b, thereby opening contacts AB of the odd-even switch 75 at time t, schematically represented in FIG. 8a before tap changer rotatable contact 21 has disengaged from stationary contact 7. SCRs 80 and 81 are gated ON when odd-even switch contacts AB open at time t, as represented in FIG. 8b so that the current flowing through the second load circuit comprising rotatable contact 21, preventive autotransformer winding 24, and conductor 27 is interrupted by SCR 80 or 81 after gating current is removed therefrom and no current is interrupted by odd-even switch contacts AB.

Opening of contacts AB deenergizes current transformer and bridge rectifiers 104 and 203. Capacitor supplies gating current to SCR 80 for an interval after contacts AB open until the charge on timing capacitor 121 decays to the point where first or second holding transistor 140 or 141 becomes biased OFF, thereby biasing shunting transistor 130 into saturation to shunt, or cut off the gating current from SCR 80 at time 1, represented in FIG. 8d after a predetermined time delay subsequent to time 1,, preferably 25 milliseconds, which is precisely controlled so that an SCR 80 or 81 interrupts the current through movable contact 21 at the following current zero represented at time t in FIG 8b rather than odd-even switch contacts AB but is sufficiently short to assure that the probability is minimal of SCRs 80 or 81 being subjected to excessive current when they are gated ON. Similarly, bridge rectifier 204 supplies gating current to SCR 81 for an interval after contacts AB open until the charge on timing capacitor 220 decays to the point where either first or second holding transistor 240 or 241 is biased OFF, thereby biasing shunting transistor 230 into saturation to shunt, or clip the gating current to SCR 81 at time t which occurs a predetermined interval after odd-even switch contacts AB open and also a predetennined interval before movable contact 21 disengages stationary contact 7.

FIG. 7c represents the condition wherein odd-even switch contact fingers B have disengaged from switch plate A to open odd even switch contacts AB and gating power has been removed from SCRs 80 and 81 so that current no longer flows through the second load circuit including tap changer movable contact 21, preventive autotransformer winding 24, and conductor 27 but does flow to load bushing L through the first load circuit including tap changer rotatable contact 20, preventive autotransformer winding 23, and contacts AD of the odd-even switch. Springs 65 continue to rotate contact carrier 37, nd movable contact 21 disengages stationary contact 7 after approximately 10 of rotation at time 1 represented in FIG. 8c. After approximately 16 of rotation of contact carrier 37, the tap changer rotatable contact 21 engages stationary contact 8 as represented in FIG. 7d and at time i represented in FIG. 80, and at this position the succeeding projection 85 of switch plate A has not yet engaged finger contacts B of the odd-even switch, thereby assuring that no arcing occurs at tap changer stationary contact 8 and rotatable contact 21.

Rotation of contact carrier through approximately 18 by loaded springs 65 engages the succeeding projection 85 on switch plate A with finger contacts B at time represented in FIG. 8a, thereby closing odd-even switch contacts AB subsequent to the tap change as represented in FIG. 7e. Closing of odd-even switch contacts AB energizes current transformer 100 and bridge rectifiers 203 and 104. Energization of bridge rectifier 104 immediately develops voltages across capacitor charging resistances 116 and 126. The voltage across resistance 116 causes gating current to flow to SCR 80 even though capacitor 115 is not charged. The potential across resistance 126 biases first and second holding transistors 140 and 141 ON. Turning of first and second holding transistors 140 and 141 ON biases shunting transistor 130 in the active region and permits gating current to flow to SCR 80. Capacitors 115 and 121 then charge through resistance 116 and 126 respectively.

Energization of bridge rectifier 203 immediately develops a voltage across capacitor charging resistance 225 which biases first and second holding transistors 240 and 241 ON even though timing capacitor 220 is not charged. Turning first and second holding transistors 240 and 241 ON reduces current flow through base resistance 234 to decrease the base-emitter bias on shunting transistor 230 so that it operates in the active region and permits gating current from bridge rectifier 204 to flow to SCR 81. Timing capacitor 220 then charges through resistance 225.

It will thus be appreciated that closure of odd-even switch contacts AB rapidly restores gating current to SCR's 80 and 81 so that they will conduct as represented in FIG. 8b if contact bounce occurs (at time t-, as represented in FIG. 8a) and will prevent erosion of the odd-even switch contacts due to arcing. The first or second load circuit being switched is always inductive because of the reactor winding 23 or 24, and it will be appreciated that the opening of a switch in an inductive circuit is much more damaging to the contacts than closure thereof. Inasmuch as SCR's and 81 commutate off at current zero subsequent to opening of the odd-even switch contacts, no inductive energy exists which can cause arcing and contact erosion. The initial current flow when odd-even contacts AB reclose is substantially zero. However, if the oddeven switch contacts should bounce after the current has built up, SCRs 80 and 81 are gated on and interrupt the current to prevent destructive arcing at the odd-even switch contacts.

As shown in FIG. 7e, contact fingers D engage the trailing edge of the same projection 85 on switch plate A that they engaged in the condition represented in FIG. 7a, thereby assuring that the flow of load current to load bushing L through the first load circuit is continuous.

Assuming that the voltage supplied to load bushing L is still beyond the voltage bandwidth, motor M will continue to drive chain 58 and sprocket wheel 57 in a direction to actuate tap changer rotatable contact 20 into engagement with stationary contact 8, and FIG. 7f illustrates the condition wherein latch 70 has been lifted by cam surface 73 from its notch 54 in index plate 53 and the loaded springs 65 have rotated contact carrier 37 and switch plate A sufficiently to disengage projection 85 on switch plate A from finger contacts D to open odd-even switch contacts AD before tap changer contact 20 has disengaged stationary contact 7. Opening of odd-even switch contacts AD deenergizes current transformer 200 and rectifier bridges 103 and 204. Bridge rectifier 104 supplies gating current over conductors 109 and 110 to SCR 80 until the charge on timing capacitor decays to the point where first or second holding transistor 140 or 141 is biased OFF, thereby biasing shunting transistor into saturation to shunt, or cut off gating current from SCR 80 a predetermined interval after contacts AD open and also assuring that odd-even switch contacts AD do not interrupt load current and that the chances of subjecting SCR 80 to a short circuit current while it is gated ON are minimal.

Similarly gating current supply capacitor 215 supplies gating current to SCR 81 over conductors 209 and 210 until the charge on timing capacitor 221 decays to the point where either first or second holding transistors 240 or 241 is biased OFF, thereby biasing shunting transistor 230 into saturation to shunt, or clip gating current from SCR 81 a predetermined interval after contacts AD open. SCR 80 commutates off after contacts AD open if the load current is negative and SCR 81 commutates off after contacts AD open if the load current is positive. Tap changer rotatable contact 20 is then free to rotate into engagement with stationary contact 8 without arcing (not shown in FIG. 7) since the first load circuit through it and conductor 28 is open at odd-even switch contacts AD. Further, since tap changer rotatable contact 20 is not carrying current when it is moved from stationary contact 7 to stationary contact 8, it is not necessary to snap it to the succeeding contact with a quick break mechanism, and rather it may be actuated at any desired speed without arcing.

The series arrangement of two avalanche diodes 82 and 83 and a power resistor 84 is preferably connected across thyristors 80 and 81 to limit high-frequency transient voltages across the thyristors.

FIG. 9 illustrates an embodiment of the invention wherein a tap change is accomplished in three discrete steps in each of which contact carrier 37 is rotated 20/3 and then latched. In the first step contact carrier 37 is rotated to the position shown in FIG. 7b where a pair of odd-even switch contacts AB is opened; in the second step contact carrier 37 is rotated to the position shown in FIG. 7d wherein tap changer movable contact 20 or 21 has been switched between adjacent stationa:

ry contacts; and in the third step contact carrier 37 is rotated to the position shown in FIG. 7e wherein odd-even switch contacts AB are reclosed. After each step springs 65 are again loaded to effect further rotation of contact carrier 37.

Index plate 53' of this embodiment has a plurality of circumferentially spaced main notches 54' which receive latch 70 to accurately position the movable tap changer rotatable contacts and 21 with respect to the stationary contacts 1-8 and N. Index plate 53' also has a first auxiliary notch 54a and a second auxiliary notch 54b between each pair of adjacent main notches 54'. Cam surfaces 73' in sprocket wheel 57' rise at a sharper angle than cam surfaces 73 in the embodiment of FIGS. 1-8 and lift latch 70 from a notch 54', 540 or 541: after sprocket wheel 57' is rotated through a similar angle than that required to free the latch in the embodiment of FIGS. 1-8. The springs (not shown) of the embodiment of FIG. 9 may be lighter than the springs 65 of the embodiment of FIGS. 1-8 and are loaded after each step to effect further rotation of actuating disk 60 and contact carrier 37.

After sprocket wheel 57' is rotated through a sufficient angle for a cam surface 73' to lift latch 70 from a main notch 54', the loaded springs 65 rotate actuating disk 60 and contact carrier 37 to the position where latch 70 falls in first auxiliary notch 54a and contact fingers B have disengaged switch plate A as shown in FIG. 7b. An energizing circuit remains completed to motor M (by means not shown) until all three steps of the tap change are accomplished, and consequently motor M continues to rotate sprocket wheel 57' after the first step and while latch 70 is in first auxiliary notch 5411, thereby again loading springs 65. After sprocket wheel 57 has rotated through a sufi'icient angle, can surface 73' lifts latch 70 from first auxiliary notch 54a and springs 65 rotate contact carrier 37 through approximately 640 during the second step to disengage tap changer movable contact 21 from stationary contact 7 and to bring it into engagement with stationary contact 8 as shown in FIG. 7d and with latch 70 engaged with second auxiliary notch 54b. After the second step, motor M continues to rotate sprocket wheel 57' until cam surface 73 lifts latch 70 from second auxiliary notch 54b to initiate the third step in which the loaded springs 65 rotate contact carrier 37 to the position shown in FIG. 7e wherein finger contacts B have engaged the succeeding radial projection 85 to reclose odd-even switch contacts AB. After the third step, electrical interlock means (not shown) operated by the actuating disk 60 open the electrical circuit to motor M so that the tap changer remains at rest if the voltage being regulated is within the desired bandwidth.

It should be understood that I do not intend to be limited to the particular embodiment of the invention shown and described for many modifications and variations thereof will be obvious to those skilled in the art.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In tap changing apparatus, the combination of a tap changer dial switch having a plurality of circumferentially spaced stationary contacts arranged in a circle, a contact support member rotatable about the axis of said circle, first and second tap changer movable contacts mounted in arcuately spaced relation on said contact support member and being adapted to sequentially engage said tap changer stationary contacts in both bridging and nonbridging tap changer positions, an odd-even switch including a conductive switch plate operatively connected to said contact support member and having a plurality of circumferentially spaced radial projections each of which subtends a central angle greater than the angle between said first and second movable tap changer contacts, stationary contact means for maintaining continuous engagement with said switch plate as said contact support member rotates, first finger contact means engaging one of said radial projections while said contact support member is at rest and also while said second tap changer movable contact is being actuated between tap changer positions upon movement of said contact support member and disengaging said one radial projection before said first movable tap changer contact disengages a stationary tap changer contact when said first movable tap changer contact is being actuated between tap changer positions upon rotation of said contact support member, and

second finger contact means engaging one of said radial projections while said contact support member is at rest and also while said first tap changer movable contact is being actuated between tap changer positions upon movement of said contact support member and disengaging said one radial projection before said second movable tap changer contact disengages a stationary tap changer contact when said second movable tap changer contact is being actuated between tap changer positions by rotation of said contact support member.

2. In the tap changing apparatus of claim 1 wherein the number of radial projections on said switch plate is equal to the number of tap changer dial switch stationary contacts and each of said dial switch stationary contacts subtends a central angle greater than the central angle subtended by each of said radial projections.

3. In the tap changing apparatus of claim 2 wherein said switch plate is mounted on said contact support member and one of said stationary finger contacts engages the corresponding said one radial projection adjacent the leading edge thereof in one direction of rotation and the other stationary finger contact engages the corresponding said one radial projection adjacent the leading edge thereof in the opposite direction of rotation when said contact support member is at rest.

4. In the tap changing apparatus of claim 3 wherein said one stationary finger contact falls between said radial projections and disengages said switch plate when said contact support means is rotated in said opposite direction and said other stationary finger contact falls between said radial projections and disengages said switch plate when said contact support means is rotated in said one direction.

5. In the tap changing apparatus of claim 1 wherein said first finger contact means engages a succeeding radial projection after said first movable tap changer contact engages a succeeding tap changer stationary contact and said second finger contact means engages a succeeding radial projection after said second movable tap changer contact engages a succeeding tap changer stationary contact.

6. In the tap changing apparatus of claim 5 and including a stationary index plate coaxial with said contact support member and having a plurality of circumferentially spaced notches, a rotatable sprocket wheel coaxial with and disposed between said contact support member and said index plate, a rotatable actuating member coaxial with and disposed between said contact support member and said sprocket wheel and being operatively connected to said contact support member, means for rotating said sprocket wheel at uniform speed, a pair of spring assemblies disposed between said sprocket wheel and said actuating member, latch means carried by said actuating member and extending through a circumferentially elongated aperture in said sprocket wheel and normally engaged in one of said notches in said index plate to prevent rotation of said actuating member until said spring assemblies are biased to a predetermined extent, and cam means carried by said sprocket wheel for lifting said latch means from said one notch after said sprocket wheel has rotated through a predetermined angle.

7. In the tap changing apparatus of claim 3 having means for rotating said contact support member to actuate said tap changer movable contacts between tap changer positions including means for latching said contact support member against movement when said tap changer movable contacts are in each of said tap changer positions, a rotatable sprocket wheel, spring means between said sprocket wheel and said contact support member, means for rotating said sprocket wheel at uniform speed to load said spring means while said contact member is latched against movement, and means for releasing said latch means after said sprocket wheel has rotated through a predetermined angle relative to said contact support member.

8. In the tap changing apparatus of claim 7 wherein said means for latching also latches said contact support member against movement in an intermediate position after rotation thereof through an angle sufficient to disengage one of said stationary finger contacts from said switch plate but insufficient to disengage one of said tap changer movement contacts from one of said tap changer dial switch stationary contacts.

9. In the tap changing apparatus of claim 8 wherein said means for releasing also frees said contact support member for movement, when it is held in said intermediate position, after said sprocket wheel has rotated through a preselected angle and wherein said means for latching also latches said contact support member against movement in a second intermediate position after rotation thereof through an angle sufficient to disengage one of said movable tap changer contacts from one of said stationary tap changer dial switch contacts but insufficient to engage said one stationary contact finger with a succeeding radial projection on said switch plate.

10. in a tap changing voltage regulator, the combination of a tap changer dial switch having a plurality of circumferentially spaced stationary contacts arranged in a circle and first and second rotatable tap changer contacts adapted to sequentially engage said stationary contacts in both bridging and nonbridging tap changer positions, a tap changer operating member coupled to said first and second rotatable contacts and being rotatable about the axis of rotation thereof, and odd-even switch means positioned adjacent said stationary dial switch contacts and having first and second normally closed pairs of contacts in series respectively with said first and second rotatable tap changer contacts and including means operatively connected to said tap changer operating member for opening said first pair of normally closed contacts during actuation of said first rotatable tap changer contact between tap changer positions and before it disengages each dial switch stationary contact and also for opening said second pair of normally closed contacts during actuation of said second rotatable tap changer contact between tap changer positions before said second rotatable tap changer contact disengages each dial switch stationary contact, said last-named means controlling the opening and closing of said first and second pairs of normally closed contacts of said odd-even switch means and reclosing said first pair after said first rotatable tap changer contact engages the succeeding tap changer stationary contact and reclosing said second pair after said second tap changer rotatable contact engages the succeeding dial tap changer stationary contact.

11. in the combination of claim 10 wherein said odd-even switch means includes a conductive switch plate disposed in a plane parallel to the plane of said tap changer dial switch stationary contacts and having a plurality of circumferentially spaced radial projections equal in number to said tap changer dial switch stationary contacts, and wherein said first and second pairs of normally closed contacts of said odd-even switch means includes first and second stationary finger contacts adapted to engage said radial projections and also includes stationary contact means in continuous engagement with said switch plate as it rotates.

12. In the combination of claim 11 wherein said first and second tap changer rotatable contacts move together and are spaced-apart arcuately, each of said tap changer dial switch stationary contacts subtends an angle greater than that between said first and second tap changer rotatable contacts, and each of said radial projections subtends an angle greater than that between said first and second tap changer rotatable contacts but smaller than that subtended by each of said dial switch stationary contacts.

13. In the combination of claim 12 wherein said tap changer operating member is a contact support member of insulating material and carries said first and second rotatable tap changer contacts and also carries said conductive switch plate, and wherein one of said finger contacts of said odd-even switch means engages one of said radial projections adjacent the leading edge thereof in one direction of rotation and the other finger contact engages one of said radial projections adjacent the leading edge thereof in the opposite direction of rotation.

14. In the combination of claim 13 wherein each of said tap changer dial switch stationary contacts subtends a central angle greater than that subtended by each of said radial projections, and wherein said one stationary finger contact falls between said radial projections and disengages said switch plate when said contact support member is rotated in said opposite direction and said other stationary contact falls between said radial projections and disengages said switch plate when said contact support member is rotated in said one direction.

15. In the combination of claim 14 and including means for actuating said first and second tap changer dial switch rotatable contacts between adjacent dial switch stationary contacts including a circular stationary index plate coaxial with said contact support member and having a plurality of circumferentially spaced main notches, latch means operatively connected to said contact support member and normally engaged within one of said main notches to prevent rotation of said contact support member and to accurately position said tap changer rotatable contacts with respect to said tap changer dial switch stationary contacts, a rotatable sprocket wheel coaxial with said contact support member and said index plate, resilient means coupling said sprocket wheel and said contact support member and adapted to be biased when said sprocket wheel is rotated and said contact support member is prevented from rotating by said latch means, and cam means for lifting said latch means from said main notch in response to a predetermined rotation of said sprocket wheel.

16. In the combination of claim 15 wherein said index plate has a first auxiliary notch between adjacent main notches adapted to receive said latch means to stop said contact support member in an intermediate position after it has rotated through a sufficient angle to open one of said pairs of oddeven switch contacts but not through a great enough angle to disengage one of said rotatable tap changer contacts from one of said dial switch stationary contacts.

17. In the combination of claim 16 wherein said index plate also has a second auxiliary notch between adjacent main notches adapted to receive said latch means to stop said contact support member after it has rotated from said intermediate position through a sufficient angle to disengage one of said rotatable tap changer contacts from one of said tap changer dial switch stationary contacts to engage it with a succeeding dial switch stationary contact but not through a large enough angle to reclose said one pair of odd-even switch contacts.

18. In the combination of claim 15 and including a rotatable actuating member coaxial with and disposed between said contact support member and said sprocket wheel and being operatively connected to said contact support member, said resilient means including a pair of spring assemblies disposed between said actuating member and said sprocket wheel, said latch means being pivotally mounted on said actuating member and extending through a circumferentially elongated aperture in said sprocket wheel to engage one of said main notches in said index plate to prevent rotation of said actuating member and said contact support member until after said predetermined rotation of said sprocket wheel.

19. in a tap changing voltage regulator, the combination of a tap changer dial switch having a plurality of circumferentially spaced stationary contacts arranged in a circle and first and second rotatable tap changer contacts adapted to sequentially engage said stationary contacts in both bridging and nonbridging tap positions, an operating member coupled to said first and second rotatable tap changer contacts and being rotatable about the axis of rotation thereof, an odd-even switch including a rotatable conductive switch plate coupled to said operating member and having a plurality of circumferentially spaced radial projections equal in number to said stationary contacts, stationary contact means for maintaining continuous engagement with said switch plate as it rotates, first stationary finger contact means engaging one of said radial projections and adapted to.disengage from it when said operating member is rotated before said first tap changer rotatable contact disengages each of said dial switch stationary contacts and to engage a succeeding radial projection after said tap changer first rotatable contact engages a succeeding dial switch stationary contact, and second finger contact means engaging another of said radial projections and adapted to disengage from it when said operating member is rotated before said second tap changer rotatable contact disengages each of said dial switch stationary contacts and to engage a succeeding radial projection after said tap changer second rotatable contact engages a succeeding dial switch stationary contact.

20. In the combination of claim 19 wherein each of said dial switch stationary contacts subtends a predetermined central angle and said tap changer first and second rotatable contacts move together and are arcuately spaced by an angle smaller than said predetermined angle, and each of said radial projections subtends a central angle which is greater than that between said tap changer first and second rotatable contacts but smaller than said predetennined central angle.

21. In the combination of claim 20 wherein said operating member is contact support member which carries said dial switch first and said second rotatable contacts and also carries said conductive switch plate of said odd-even switch, and one of said contact finger means of said odd-even switch engages one of said radial projections adjacent the leading edge thereof and the other of said contact finger means engages another of said radial projections adjacent the trailing edge thereof relative to both directions of rotation of said contact support member when said conductive switch plate is at rest.

22. In tap changing apparatus, the combination of a tap changer dial switch having circumferentially spaced stationary contacts arranged in a circle, a contact support member rotatable about the axis of said circle, first and second tap changer movable contacts mounted in arcuately spaced relation on said contact support member and being adapted to sequentially engage said stationary contacts in both bridging and nonbridging positions, an odd-even switch including a conductive switch plate operatively connected to said contact support member and having circumferentially spaced radial projections each of which subtends a central angle greater than the angle between said first and second movable tap changers contacm, stationary contact means for maintaining continuous engagement with said switch plate as said contact support member rotates, and first and second finger contacts each of which normally engages one of said radial projections and is positioned so that upon rotation of said contact carrier in either direction said first finger contact falls between said radial projections and disengages said switch plate before said first tap changer movable contact disengages a stationary tap changer contact and said second finger contact falls between said radial projections and disengages said switch plate before said second movable tap changer contact disengages a stationary tap changer contact.

23. In the tap changing apparatus of claim 22 wherein each of said tap changer stationary contacts subtends a central angle greater than that subtended by each of said radial projections and said first finger contact engages a succeeding radial projection after said first movable tap changer contact engages a succeeding stationary tap changer contact and said second finger contact engages a succeeding radial projection after said second movable tap changer contact engages a succeeding stationary tap changer contact. 

1. In tap changing apparatus, the combination of a tap changer dial switch having a plurality of circumferentially spaced stationary contacts aRranged in a circle, a contact support member rotatable about the axis of said circle, first and second tap changer movable contacts mounted in arcuately spaced relation on said contact support member and being adapted to sequentially engage said tap changer stationary contacts in both bridging and nonbridging tap changer positions, an odd-even switch including a conductive switch plate operatively connected to said contact support member and having a plurality of circumferentially spaced radial projections each of which subtends a central angle greater than the angle between said first and second movable tap changer contacts, stationary contact means for maintaining continuous engagement with said switch plate as said contact support member rotates, first finger contact means engaging one of said radial projections while said contact support member is at rest and also while said second tap changer movable contact is being actuated between tap changer positions upon movement of said contact support member and disengaging said one radial projection before said first movable tap changer contact disengages a stationary tap changer contact when said first movable tap changer contact is being actuated between tap changer positions upon rotation of said contact support member, and second finger contact means engaging one of said radial projections while said contact support member is at rest and also while said first tap changer movable contact is being actuated between tap changer positions upon movement of said contact support member and disengaging said one radial projection before said second movable tap changer contact disengages a stationary tap changer contact when said second movable tap changer contact is being actuated between tap changer positions by rotation of said contact support member.
 2. In the tap changing apparatus of claim 1 wherein the number of radial projections on said switch plate is equal to the number of tap changer dial switch stationary contacts and each of said dial switch stationary contacts subtends a central angle greater than the central angle subtended by each of said radial projections.
 3. In the tap changing apparatus of claim 2 wherein said switch plate is mounted on said contact support member and one of said stationary finger contacts engages the corresponding said one radial projection adjacent the leading edge thereof in one direction of rotation and the other stationary finger contact engages the corresponding said one radial projection adjacent the leading edge thereof in the opposite direction of rotation when said contact support member is at rest.
 4. In the tap changing apparatus of claim 3 wherein said one stationary finger contact falls between said radial projections and disengages said switch plate when said contact support means is rotated in said opposite direction and said other stationary finger contact falls between said radial projections and disengages said switch plate when said contact support means is rotated in said one direction.
 5. In the tap changing apparatus of claim 1 wherein said first finger contact means engages a succeeding radial projection after said first movable tap changer contact engages a succeeding tap changer stationary contact and said second finger contact means engages a succeeding radial projection after said second movable tap changer contact engages a succeeding tap changer stationary contact.
 6. In the tap changing apparatus of claim 5 and including a stationary index plate coaxial with said contact support member and having a plurality of circumferentially spaced notches, a rotatable sprocket wheel coaxial with and disposed between said contact support member and said index plate, a rotatable actuating member coaxial with and disposed between said contact support member and said sprocket wheel and being operatively connected to said contact support member, means for rotating said sprocket wheel at uniform speed, a pair of spring assemblies disposed between said sprocket wheel and said Actuating member, latch means carried by said actuating member and extending through a circumferentially elongated aperture in said sprocket wheel and normally engaged in one of said notches in said index plate to prevent rotation of said actuating member until said spring assemblies are biased to a predetermined extent, and cam means carried by said sprocket wheel for lifting said latch means from said one notch after said sprocket wheel has rotated through a predetermined angle.
 7. In the tap changing apparatus of claim 3 having means for rotating said contact support member to actuate said tap changer movable contacts between tap changer positions including means for latching said contact support member against movement when said tap changer movable contacts are in each of said tap changer positions, a rotatable sprocket wheel, spring means between said sprocket wheel and said contact support member, means for rotating said sprocket wheel at uniform speed to load said spring means while said contact member is latched against movement, and means for releasing said latch means after said sprocket wheel has rotated through a predetermined angle relative to said contact support member.
 8. In the tap changing apparatus of claim 7 wherein said means for latching also latches said contact support member against movement in an intermediate position after rotation thereof through an angle sufficient to disengage one of said stationary finger contacts from said switch plate but insufficient to disengage one of said tap changer movement contacts from one of said tap changer dial switch stationary contacts.
 9. In the tap changing apparatus of claim 8 wherein said means for releasing also frees said contact support member for movement, when it is held in said intermediate position, after said sprocket wheel has rotated through a preselected angle and wherein said means for latching also latches said contact support member against movement in a second intermediate position after rotation thereof through an angle sufficient to disengage one of said movable tap changer contacts from one of said stationary tap changer dial switch contacts but insufficient to engage said one stationary contact finger with a succeeding radial projection on said switch plate.
 10. In a tap changing voltage regulator, the combination of a tap changer dial switch having a plurality of circumferentially spaced stationary contacts arranged in a circle and first and second rotatable tap changer contacts adapted to sequentially engage said stationary contacts in both bridging and nonbridging tap changer positions, a tap changer operating member coupled to said first and second rotatable contacts and being rotatable about the axis of rotation thereof, and odd-even switch means positioned adjacent said stationary dial switch contacts and having first and second normally closed pairs of contacts in series respectively with said first and second rotatable tap changer contacts and including means operatively connected to said tap changer operating member for opening said first pair of normally closed contacts during actuation of said first rotatable tap changer contact between tap changer positions and before it disengages each dial switch stationary contact and also for opening said second pair of normally closed contacts during actuation of said second rotatable tap changer contact between tap changer positions before said second rotatable tap changer contact disengages each dial switch stationary contact, said last-named means controlling the opening and closing of said first and second pairs of normally closed contacts of said odd-even switch means and reclosing said first pair after said first rotatable tap changer contact engages the succeeding tap changer stationary contact and reclosing said second pair after said second tap changer rotatable contact engages the succeeding dial tap changer stationary contact.
 11. In the combination of claim 10 wherein said odd-even switch means includes a conductive switCh plate disposed in a plane parallel to the plane of said tap changer dial switch stationary contacts and having a plurality of circumferentially spaced radial projections equal in number to said tap changer dial switch stationary contacts, and wherein said first and second pairs of normally closed contacts of said odd-even switch means includes first and second stationary finger contacts adapted to engage said radial projections and also includes stationary contact means in continuous engagement with said switch plate as it rotates.
 12. In the combination of claim 11 wherein said first and second tap changer rotatable contacts move together and are spaced-apart arcuately, each of said tap changer dial switch stationary contacts subtends an angle greater than that between said first and second tap changer rotatable contacts, and each of said radial projections subtends an angle greater than that between said first and second tap changer rotatable contacts but smaller than that subtended by each of said dial switch stationary contacts.
 13. In the combination of claim 12 wherein said tap changer operating member is a contact support member of insulating material and carries said first and second rotatable tap changer contacts and also carries said conductive switch plate, and wherein one of said finger contacts of said odd-even switch means engages one of said radial projections adjacent the leading edge thereof in one direction of rotation and the other finger contact engages one of said radial projections adjacent the leading edge thereof in the opposite direction of rotation.
 14. In the combination of claim 13 wherein each of said tap changer dial switch stationary contacts subtends a central angle greater than that subtended by each of said radial projections, and wherein said one stationary finger contact falls between said radial projections and disengages said switch plate when said contact support member is rotated in said opposite direction and said other stationary contact falls between said radial projections and disengages said switch plate when said contact support member is rotated in said one direction.
 15. In the combination of claim 14 and including means for actuating said first and second tap changer dial switch rotatable contacts between adjacent dial switch stationary contacts including a circular stationary index plate coaxial with said contact support member and having a plurality of circumferentially spaced main notches, latch means operatively connected to said contact support member and normally engaged within one of said main notches to prevent rotation of said contact support member and to accurately position said tap changer rotatable contacts with respect to said tap changer dial switch stationary contacts, a rotatable sprocket wheel coaxial with said contact support member and said index plate, resilient means coupling said sprocket wheel and said contact support member and adapted to be biased when said sprocket wheel is rotated and said contact support member is prevented from rotating by said latch means, and cam means for lifting said latch means from said main notch in response to a predetermined rotation of said sprocket wheel.
 16. In the combination of claim 15 wherein said index plate has a first auxiliary notch between adjacent main notches adapted to receive said latch means to stop said contact support member in an intermediate position after it has rotated through a sufficient angle to open one of said pairs of odd-even switch contacts but not through a great enough angle to disengage one of said rotatable tap changer contacts from one of said dial switch stationary contacts.
 17. In the combination of claim 16 wherein said index plate also has a second auxiliary notch between adjacent main notches adapted to receive said latch means to stop said contact support member after it has rotated from said intermediate position through a sufficient angle to disengage one of said rotatable tap changer contacts from one of said tap Changer dial switch stationary contacts to engage it with a succeeding dial switch stationary contact but not through a large enough angle to reclose said one pair of odd-even switch contacts.
 18. In the combination of claim 15 and including a rotatable actuating member coaxial with and disposed between said contact support member and said sprocket wheel and being operatively connected to said contact support member, said resilient means including a pair of spring assemblies disposed between said actuating member and said sprocket wheel, said latch means being pivotally mounted on said actuating member and extending through a circumferentially elongated aperture in said sprocket wheel to engage one of said main notches in said index plate to prevent rotation of said actuating member and said contact support member until after said predetermined rotation of said sprocket wheel.
 19. In a tap changing voltage regulator, the combination of a tap changer dial switch having a plurality of circumferentially spaced stationary contacts arranged in a circle and first and second rotatable tap changer contacts adapted to sequentially engage said stationary contacts in both bridging and nonbridging tap positions, an operating member coupled to said first and second rotatable tap changer contacts and being rotatable about the axis of rotation thereof, an odd-even switch including a rotatable conductive switch plate coupled to said operating member and having a plurality of circumferentially spaced radial projections equal in number to said stationary contacts, stationary contact means for maintaining continuous engagement with said switch plate as it rotates, first stationary finger contact means engaging one of said radial projections and adapted to disengage from it when said operating member is rotated before said first tap changer rotatable contact disengages each of said dial switch stationary contacts and to engage a succeeding radial projection after said tap changer first rotatable contact engages a succeeding dial switch stationary contact, and second finger contact means engaging another of said radial projections and adapted to disengage from it when said operating member is rotated before said second tap changer rotatable contact disengages each of said dial switch stationary contacts and to engage a succeeding radial projection after said tap changer second rotatable contact engages a succeeding dial switch stationary contact.
 20. In the combination of claim 19 wherein each of said dial switch stationary contacts subtends a predetermined central angle and said tap changer first and second rotatable contacts move together and are arcuately spaced by an angle smaller than said predetermined angle, and each of said radial projections subtends a central angle which is greater than that between said tap changer first and second rotatable contacts but smaller than said predetermined central angle.
 21. In the combination of claim 20 wherein said operating member is contact support member which carries said dial switch first and said second rotatable contacts and also carries said conductive switch plate of said odd-even switch, and one of said contact finger means of said odd-even switch engages one of said radial projections adjacent the leading edge thereof and the other of said contact finger means engages another of said radial projections adjacent the trailing edge thereof relative to both directions of rotation of said contact support member when said conductive switch plate is at rest.
 22. In tap changing apparatus, the combination of a tap changer dial switch having circumferentially spaced stationary contacts arranged in a circle, a contact support member rotatable about the axis of said circle, first and second tap changer movable contacts mounted in arcuately spaced relation on said contact support member and being adapted to sequentially engage said stationary contacts in both bridging and nonbridging positions, an odd-even switch including a conductive switch plAte operatively connected to said contact support member and having circumferentially spaced radial projections each of which subtends a central angle greater than the angle between said first and second movable tap changers contacts, stationary contact means for maintaining continuous engagement with said switch plate as said contact support member rotates, and first and second finger contacts each of which normally engages one of said radial projections and is positioned so that upon rotation of said contact carrier in either direction said first finger contact falls between said radial projections and disengages said switch plate before said first tap changer movable contact disengages a stationary tap changer contact and said second finger contact falls between said radial projections and disengages said switch plate before said second movable tap changer contact disengages a stationary tap changer contact.
 23. In the tap changing apparatus of claim 22 wherein each of said tap changer stationary contacts subtends a central angle greater than that subtended by each of said radial projections and said first finger contact engages a succeeding radial projection after said first movable tap changer contact engages a succeeding stationary tap changer contact and said second finger contact engages a succeeding radial projection after said second movable tap changer contact engages a succeeding stationary tap changer contact. 